In a yarn-feeding apparatus of this type, which is known from European Patent Application No. EPA 80 10 6719 and its corresponding U.S. application Ser. No. 269,061 as filed May 18, 1981, now U.S. Pat. No. 4,386,508, the yarn-guiding elements are moved into nonfeeding areas by magnets, while their movement into feeding areas is effected by increased yarn tensions caused by the operation of the fingers in the circular mechanism. However, yarn-feeding apparati are also known in which the movement of the yarn-guiding elements is effected exclusively by tension changes in the yarn caused by the operation of the fingers of the circular mechanism. Each yarn-transporting element is a constantly driven belt which runs over a freely rotatable roller. As soon as the yarn-guiding element moves the yarn under the belt, same carries along the yarn and moves it to the associated finger. Vice versa during a movement of the yarn-guiding element into the nonfeeding area, the yarn is moved out from under the belt and stops. In the lastmentioned conventional yarn-feeding apparatus, it is necessary that the path of movement of the guiding elements is sufficiently long so that the yarn-guiding element compensates for unavoidable trailing movement of the yarn, which movement results due to the inertia of the yarn mass and the yarn elasticity, when the earlier effective acceleration is suddenly reduced by the yarn-transporting element. The position sensor which is connected with the yarn-guiding element emits a signal which indicates a yarn breakage in an end position of the yarn-guiding element in the nonfeeding area, which end position can be reached by the yarn-guiding element only when in addition to the normal trailing movement due to the yarn breakage the yarn-guiding element was able to move on. The knitting machine is turned off with this signal. In all other positions in the nonfeeding area and in the feeding area, the position sensor does not emit a signal. It is thereby disadvantageous that a signal cannot be produced with the position sensor when a yarn error occurs for which the knitting machine does not use the amount of yarn fed to it in an orderly manner, for example due to an interference in the path of the yarn to the circular mechanism and to the knitting needles or because of dirty yarn eyelets or an incorrect knitting machine adjustment. During such errors, the tension in the yarn which comes from the yarn-feeding apparatus is reduced, which causes the guiding element to move the yarn out of the feeding area, but without thereby reaching its end position. Shortly thereafter the yarn tension again increases, since the positive feed was ended, which causes the yarn-guiding element to move the yarn again into the feeding area. The position sensor does not react to this yarn error; rather, the guide element can reach its end position in which a shut-off signal is produced by the position sensor when the voltage in the in-feeding yarn decreases slowly, if such yarn was not processed for a longer period of time and due to vibrations or air streams starts to sag. Such an error is then difficult to find, in particular if the contact function of the position sensor is not particularly sensitive, and can cause a considerable standstill time for the knitting machine. A further yarn error exists when in the circular mechanism, after a color change, two yarns are processed simultaneously. This yarn error is also not indicated by the position sensor, since it does not reach its end position.
A basic purpose of the invention is to provide a yarn-feeding apparatus of the abovementioned type which, in response to actual and serious yarn errors, reliably and quickly turns off the knitting machine.
Each yarn-guiding element produces in the construction a signal when the yarn comes into engagement with the transporting element or out of engagement with the transporting element, and does not wait until the end position. The signals which are emitted by all position sensors and which are processed in the logic switching circuit form in the various operating conditions different signal combinations, from which the logic switching circuit can detect whether or not an error condition exists which requires the knitting machine to be turned off. The processing of the signals and the evaluation of the signal combination involves the recognition that, during orderly operation with no yarn breakage or yarn error, only one single yarn may be processed, which must then be fed positively, so that more than one positively fed yarn and also no positively fed yarn at all can only mean a yarn error. Incorrect errors are ignored, since it is of no importance for the signal emission of the positive sensor whether the yarn-guiding element is in the end position or not, because of suitable signal is already being formed as soon as the yarn moves out of or into the area of engagement of the transporting element. Due to the fact that, in this relatively narrow range between positive feed and no feed, the signal change of the position sensor is effected, it is possible to form with all provided position sensors practically delay-free combinations out of such signals, which in certain combinations are representative of a yarn error, and due to this the switching unit then quickly stops the knitting machine suddenly so that damage in the goods is kept as small as possible. When in a knitting machine which is equipped with such a yarn-feeding apparatus an orderly color change takes place with an overlap during which two yarns are processed simultaneously, then the knitting machine, does not not need to be turned off since no actual yarn error exists. No significant measures are needed for this. It is only important that the type and manner of forming signals by the position sensors makes it possible for the switching circuit to recognize actual errors immediately in connection with the help of the signal combinations and to turn off the knitting machine.